public interface Filter {

    /** 
    * Called by the web container to indicate to a filter that it is being placed into
    * service. The servlet container calls the init method exactly once after instantiating the
    * filter. The init method must complete successfully before the filter is asked to do any
    * filtering work. <br><br>

        * The web container cannot place the filter into service if the init method either<br>
        * 1.Throws a ServletException <br>
        * 2.Does not return within a time period defined by the web container 
    */
    public void init(FilterConfig filterConfig) throws ServletException;
    
    
    /**
    * The <code>doFilter</code> method of the Filter is called by the container
    * each time a request/response pair is passed through the chain due
    * to a client request for a resource at the end of the chain. The FilterChain passed in to this
    * method allows the Filter to pass on the request and response to the next entity in the
    * chain.<p>
    * A typical implementation of this method would follow the following pattern:- <br>
    * 1. Examine the request<br>
    * 2. Optionally wrap the request object with a custom implementation to
    * filter content or headers for input filtering <br>
    * 3. Optionally wrap the response object with a custom implementation to
    * filter content or headers for output filtering <br>
    * 4. a) <strong>Either</strong> invoke the next entity in the chain using the FilterChain object (<code>chain.doFilter()</code>), <br>   
    ** 4. b) <strong>or</strong> not pass on the request/response pair to the next entity in the filter chain to block the request processing<br>
    ** 5. Directly set headers on the response after invocation of the next entity in the filter chain.
    **/
    public void doFilter ( ServletRequest request, ServletResponse response, FilterChain chain ) throws IOException, ServletException;

    /**
    * Called by the web container to indicate to a filter that it is being taken out of service. This 
    * method is only called once all threads within the filter's doFilter method have exited or after
    * a timeout period has passed. After the web container calls this method, it will not call the
    * doFilter method again on this instance of the filter. <br><br>
    * 
        * This method gives the filter an opportunity to clean up any resources that are being held (for
    * example, memory, file handles, threads) and make sure that any persistent state is synchronized
    * with the filter's current state in memory.
    */

    public void destroy();


}


public interface FilterChain {
    
    /**
    * Causes the next filter in the chain to be invoked, or if the calling filter is the last filter
    * in the chain, causes the resource at the end of the chain to be invoked.
    *
    * @param request the request to pass along the chain.
    * @param response the response to pass along the chain.
    *
    * @since 2.3
    */
    
    public void doFilter ( ServletRequest request, ServletResponse response ) throws IOException, ServletException;

}


public interface FilterConfig {

    /** 
    * Returns the filter-name of this filter as defined in the deployment descriptor. 
    */
    
    public String getFilterName();


 /**
     * Returns a reference to the {@link ServletContext} in which the caller
     * is executing.
     *
     *
     * @return       a {@link ServletContext} object, used
     *           by the caller to interact with its servlet 
     *                  container
     * 
     * @see      ServletContext
     *
     */

    public ServletContext getServletContext();
    
    /**
     * Returns a <code>String</code> containing the value of the 
     * named initialization parameter, or <code>null</code> if 
     * the parameter does not exist.
     *
     * @param name   a <code>String</code> specifying the name
     *           of the initialization parameter
     *
     * @return       a <code>String</code> containing the value 
     *           of the initialization parameter
     *
     */

    public String getInitParameter(String name);


    /**
     * Returns the names of the filter's initialization parameters
     * as an <code>Enumeration</code> of <code>String</code> objects, 
     * or an empty <code>Enumeration</code> if the filter has
     * no initialization parameters.
     *
     * @return       an <code>Enumeration</code> of <code>String</code> 
     *           objects containing the names of the filter's 
     *           initialization parameters
     *
     *
     *
     */

    public Enumeration getInitParameterNames();




}